The present invention relates to a flow control mechanism for providing a variable fluid flow and, in particular, relates to a mechanism which operates to control flow in response to an electrical signal.
Obviously, fluid flow from a pump can be controlled by a variable orifice, and a valve which maintains a constant pressure drop across the orifice.
In some applications the valve for maintaining the constant pressure drop across the orifice can be a bypass valve. Commonly a bypass valve ports fluid from the pump outlet directly to the pump inlet. When the orifice area is varied, the change in pressure drop across the orifice is sensed and the bypass valve moves to bypass more or less fluid to thereby maintain the constant pressure drop across the orifice.
Coffman, U.S. Pat. No. 4,099,893 also assigned to TRW Inc. discloses a flow control mechanism as referred to above. It includes a pintle member and a valve seat which together define a variable orifice. The pressure drop across the orifice controls a bypass valve. The pintle member is positioned by an electrically actuated rack and pinion gear arrangement. Obviously, the position of the pintle member depends upon the time interval during which the motor is energized, the inertia of the motor and gear arrangement, etc. Thus accurate control of fluid flow is delayed because of the amount of time required to position the pintle member. Also, it is difficult to accurately position the pintle. This is due to the pintle position being dependent upon the time interval of operation of the motor, which time interval is cumbersome to control. Furthermore, the position of the pintle member may not be known when an adjustment is initiated, since the pintle member does not inherently return to a predetermined position either upon start-up or shutdown.
The flow control mechanism of the present invention is particularly useful for regulating the flow from a constant displacement pump, such as a gear pump. The flow control mechanism may be constructed together with the pump in a single integral unit or may be a separate unit. Basically, the flow control mechanism comprises a variable orifice and a bypass valve which is operated by a pressure differential across the variable orifice. The bypass valve allows a pintle valve to maintain a constant pressure drop across an orifice in the outlet of the pump and thus a desired constant flow to the system. If the orifice size is changed, the rate of flow to the system is changed.
In accordance with the present invention, the variable orifice comprises a valve seat and a pintle which is freely movable relative to the valve seat. The pintle has first and second ends. The first end of the pintle is located proximate the valve seat for restricting fluid flow therethrough to an extent which depends upon the position of the pintle member with respect to the valve seat. Fluid pressure acts on the area of the first end of the pintle. This force urges the pintle away from the seat. A pressure chamber is provided adjacent the second end of the pintle and is supplied with fluid. The pressure in the chamber acts against the area of the second end of the pintle to create a force opposing the force acting against the first end of the pintle. A fluid discharge passage allows fluid to flow from the pressure chamber A pilot member is positionable relative to the discharge passage for regulating the pressure build-up within the chamber. Only a relatively small force is needed to move the pilot member and achieve a proportionate movement of the pintle.
The pilot member is preferably positioned relative to the passage by an electrically operated solenoid having an armature which is operatively associated with the pilot member. The position of the armature, and thus the position of the pilot member, is determined by the electromagnetic force applied to the armature by the coil of the solenoid, acting in opposition to a spring bias. Since the magnitude of this force will depend upon the amplitude of the electrical signal which controls the solenoid, the position of the pilot member with respect to the discharge passage will be directly dependent upon the amplitude of this electrical signal.
The present invention is an improvement in the structure disclosed in U.S. Pat. No. 4,099,893. The pintle member in the present invention is positioned immediately upon the solenoid being energized. Thus, positioning of the pintle is not time dependent, as when the pintle is moved by a motor as in U.S. Pat. No. 4,099,893. Further, in the present invention, the pintle returns to a predetermined position when the solenoid is deenergized. Accordingly, the position of the pintle is known when adjustment is initiated by energization of the solenoid.